<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>box-shadow绘制边框</title>
</head>
<style>

    .container {
        width: 100vw;
        height: 100vh;
        display: grid;
        align-items: center;
        justify-content: center;
    }

    input {
        border: none;
        outline: none;
        border-bottom: 1px solid blue;
        padding-bottom: 5px;
    }

    input:focus {
        border: none;
        padding-bottom: 0px;
       /* box-shadow: 1px 1px lightskyblue,
        1px -1px lightskyblue,
        -1px 1px lightskyblue,
        -1px -1px lightskyblue;*/
        box-shadow: 0 0 0 1px lightskyblue;
    }
</style>
<body>
<div class="container">

    <form action="">
        <input type="text" placeholder="想说点啥">
    </form>
</div>

</body>
</html>